1. Field of the Invention
The present invention generally relates to wireless communications modems and, more particularly, to an apparatus and method for ramping up the transmitter power of a wireless modem.
2. Description of the Related Art
Wireless modems generally modulate an RF carrier with a stream of data packets, then amplify the modulated carrier for transmission from an internal antenna. When a transmitter carrier is keyed on or first brought up to full power, the transmitter is essentially being amplitude modulated from an off state to a full powered on state. The quicker the transmitter power is ramped up, the wider the spread of spurious emissions, called splatter, occur around the carrier frequency. Splatter results in the placement of unwanted energy into adjacent channels in the ISM band, 900 MHz (Industrial, Scientific and Medical band).
Currently, wireless modems use hardware to reduce the level of the out of band energy, thereby lessening the chances of interference. The system controls the amount of splatter by controlling the speed at which power is ramped up in the transmitting device. One modem accomplishes this through the hardware by sensing the current supplied to a power amplifier in the transmitter during the power-up and power-down states and controlling the power gain of the power amplifier (PA) using a negative bias.
As transmission rates continue to increase, the specified amount of turn around time is shortening. The combination of high data rate and short turn around time requires that the transmitted packet have a minimum amount of overhead, thus ramping up the modem as quickly as possible with a minimum amount of splatter.
A disadvantage of the aforementioned hardware driven technique is that the degree of controlling the rate of the PA power gain, and thus the resultant splatter, is very limited. Typically in a transmitter, the output power is ramped up by increasing the gain of the PA or by applying power supply bias to the PA while driven with the carrier, as mentioned above. The shape of the ramp up curve is determined by some resistor/capacitor RC time constant and the transfer function of the PA. The amount of splatter that appears in adjacent channels, as measured by the amount of power in those channels, is proportional to the time rate of change of the power level (slope of the envelope shape) and the PA output power. A very steep envelope slope at low power levels will not cause sufficient splatter but the same slope near the PA's maximum power will cause considerable splatter. For a given total ramp up time, the amount of splatter can be minimized by ramping up the output power quickly at low power levels and then reduce the rate as full output power is approached. An RC time constant may nominally tend to do this, but the degree of controlling the rate of power increase in relation to the time rate of change of the power level is difficult and very limited when relying mainly on the hardware.
Some of the limitations of the prior art lie in its inability to effectively shorten the ramp up time while maintaining a low level of splatter between the transmitting channel and adjacent channels. Control of the ramp up curve is difficult to achieve with hardware alone, thus resulting in overly complex digital ramp circuitry which still has not been able to meet the needs required by advancing transmission speeds.